Three people, including an eight-year-old boy, were killed, while at least 17 people are reported to be in a critical condition.

Designed to propel shrapnel

Images in the immediate aftermath showed people being carried away on stretchers -- one man in a wheelchair had blood all over his face and legs.

Photos: Deadly attack at Boston Marathon 55 photos

Photos: Deadly attack at Boston Marathon55 photos

Deadly attack at Boston Marathon – An injured man is loaded into an ambulance after two bombs went off near the finish line of the Boston Marathon on April 15, 2013. Three people were killed and at least 264 were injured.

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Photos: Deadly attack at Boston Marathon55 photos

A man comforts a victim on the sidewalk at the scene of the first explosion.

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While many patients were treated for cuts and scrapes, doctors have also been "pulling ball bearings out of people in the emergency room," suggesting the bombs were designed to propel shrapnel, according to one terrorism expert briefed on the Boston blasts, though CNN is yet to confirm this.

Medical teams have also carried out at least 10 amputations and treated many leg injuries, suggesting the device was low to the ground, according to CNN's Deborah Feyerick.

Among the 28 people taken to the city's Brigham and Women's Hospital, the most common types of injuries are to the bone and tissue, hospital spokesman Tom Langford told CNN.

He said nine of those patients have potentially limb-threatening wounds. A 3-year-old victim was transferred to Boston Children's Hospital for treatment.

Amputated limbs

Dr. Peter Fagenholz, a trauma surgeon at Massachusetts General Hospital, told reporters he treated many of the 29 patients who arrived at the hospital for shrapnel wounds.

"Many of the victims were hit with "a lot of small, metal debris," he said.

"Some people have asked already whether they were BBs or parts of bombs," he said, referring to earlier reports that ball bearings have been pulled from victims. "We can't say whether they were placed their intentionally or whether they were just part of the environment."

He said the most serious wounds "have been combined, complex lower injuries that involve blood vessels, bone and tissue." Some of the patients had to have limbs amputated, Fagenholz added.

"A number of patients will require repeat operations and serial operations over the next couple of days," he said, adding that he had been in surgery for almost 14 hours.

Describing the horror

Theresa Panter, who had been running in the marathon, described the scene as she approached the finishing line.

"When I heard the bomb and saw the reaction of the spectators, I was just alarmed. Then I was pushed back by a spectator and then a Boston Athletic official -- he grabbed a bunch of us and pushed us back. It was pretty upsetting."

Her husband, Dr. Allan Panter, was in the crowd and described how he ended up tending to people on the streets.

"I saw at least six to seven people down next to me -- they protected me from the blast. One lady expired, one gentleman lost both his limbs, his lower extremities. Most of the injuries were mainly lower extremity injuries.

"I could not figure out why the young lady had expired, I could not find any injury on her thorax."